Microwave slot antennas have been used as stand-alone antennas and as elements of antenna arrays. They generally comprise a metal ground plane, a dielectric board, a metal feed line and a metal cover, the radiating slot being cut or etched in the ground plane at an angle of 0.degree. to 90.degree. to the line. Such slots present series impedance to the feed line.
In the prior art, the slot antenna has been considered narrow band in nature. In view of this assumed property, slot antenna designs have been aimed at achieving good impedance match over a narrow frequency band, of 10% (for wide slots) or less. This match is commonly realized by cancelling the reactive portion of the impedance by a quarter wavelength open circuit stub at the load end of the radiator, extending beyond the slot. Impedance match at a single frequency was achieved by dislocation of the feed point from the center of the slot (offset-fed slot). Thus matched, highly efficient operation of the slot radiator was achieved, however, only within a narrow frequency band. Existing matching networks have been narrow band by nature. Impedance transformers along the feeding line at the generator side, if present, have been a part of a power dividing network, rather than the antenna element itself. An example of a power dividing network is the Wilkinson type, wherein a 50- ohm line is divided into two 100-ohms lines followed by impedance transformers for transforming the impedance back to the 50-ohm level.